Known solutions for integrating in a single semiconductor chip MOS devices with different threshold voltages provide for varying the thickness of the gate oxides, or the dopant concentrations in the channel regions of the MOS transistors. For example, if horizontal MOS transistors with different threshold voltages are to be integrated in the same chip, it is necessary to perform as many selective dopant implantations as the different values of the threshold voltages to be obtained, with each implantation having a respective dopant dose and involving only the channel regions of those MOS transistors having the same threshold voltage. Alternatively, the horizontal MOS transistors can be formed inside respective doped semiconductor wells, each well having a different dopant concentration, which are formed in a common semiconductor substrate.
If instead of acting on the dopant concentrations in the channel regions one acts on the thickness of the gate oxides, as many steps of growth and selective removal of oxide layers are necessary as the different values of the threshold voltages to be obtained.
Similar considerations are valid also for vertical MOS transistors. The known processes for the fabrication of vertical MOS transistors provides for the formation, on a lightly doped drain layer, for example, of N-type, of a gate electrode, and the selective introduction in the drain layer of a dopant (of P-type in this example) using the gate electrode as a mask for the formation of the so-called body regions of the transistor; such body regions extend under the gate electrode to form a channel region of the transistor. In order to integrate in a single chip vertical MOS transistors with different threshold voltages, it is necessary to perform, for each transistor, a dedicated dopant implantation so as to form body regions with different dopant concentrations or, if one wishes to act on the thickness of the gate oxides, to provide as many steps of growth and selective removal of oxide layers as the different values of the threshold voltage to be obtained.
This obviously makes the manufacturing processes longer, more complex and thus more costly.